Fix for broken connection-pool in Rails < 3.3.0

connection_pool_fix.rb

# Monkey patch ConnectionPool#checkout to avoid database connection timeouts
# Source: https://github.com/rails/rails/issues/2547
# For Rails 3.2.0 and upper, You need to check if the pool error still occurs
if Rails.version < "3.3.0"
  class ActiveRecord::ConnectionAdapters::ConnectionPool
    def checkout
      # Checkout an available connection
      start_at = Time.now
      @connection_mutex.synchronize do
        loop do
          conn = try_to_obtain_connection
          return conn if conn

          should_not_be_timeout!(start_at)

          # No connections available; wait for one
          is_timeout = !@queue.wait(@timeout) # mutex is lost

          # mutex is obtained again
          if is_timeout
            # try looting dead threads
            clear_stale_cached_connections!
            raise_timeout_error! unless can_obtain_connection?
          end
        end
      end
    end

    synchronize :connection, :release_connection, :clear_stale_cached_connections!, :with => :@connection_mutex

    private
    def can_obtain_connection?
      @size > @checked_out.size
    end

    def should_not_be_timeout!(start_at)
      return if (Time.now - start_at) < @timeout
      raise_timeout_error!
    end

    def try_to_obtain_connection
      if @checked_out.size < @connections.size
        checkout_existing_connection
      elsif @connections.size < @size
        checkout_new_connection
      end
    end

    def raise_timeout_error!
      raise ActiveRecord::ConnectionTimeoutError, "could not obtain a database connection#{" within #{@timeout} seconds" if @timeout}.  The max pool size is currently #{@size}; consider increasing it."
    end
  end
end